Heart valve regurgitation is a condition whereby the heart valve does not seal completely as a result of disease or injury, and may have fatal consequences. Valve stenosis is a condition where the valve is narrowed and cannot open normally. Whilst valve stenosis can be treated by valvuloplasty (by balloon dilatation), this often results in the valve leaking and may require valve replacement. Aortic valvuloplasty is generally not a very effective or durable treatment for aortic stenosis.
Malfunctioning heart valves have typically been replaced with mechanical or biologic heart valve prostheses using highly invasive open-heart surgical techniques. Surgical mitral valve replacement is quite invasive and cannot be performed on many sick patients with severe mitral regurgitation. This procedure often results in resection of the anterior leaflet of the mitral valve which could lead to further left ventricular dysfunction. Whilst there has been some success in developing replacement stent based aortic valve prostheses for delivery via percutaneous catheter-based methods, these techniques have not been particularly successful when applied to mitral valve prostheses.
Mitral valve replacement is firstly made difficult as a result of the anatomy of the mitral valve, and particularly that of the mitral valve annulus in which the mitral valve leaflets are located. The mitral valve annulus is typically very distorted, and of unpredictable and non-uniform geometries, as compared to the relatively uniform aortic valve annulus. This unpredictable anatomy makes it difficult to design a pre-constructed mitral valve prosthesis that would fit the mitral valve annulus in a satisfactory manner for safe, stable and meticulous deployment.
Further, unlike the aortic valve annulus which is entirely surrounded by muscular tissue, the mitral valve annulus is bounded by muscular tissue on the outer wall only, with the inner side of the mitral valve annulus being bounded by a thin vessel wall which separates the mitral valve annulus and the aortic outflow tract. As a result, the mitral valve annulus cannot be subjected to any significant radial forces, as would be typical with an expanding stent type of valve prosthesis, as such radial forces would tend to collapse the aortic outflow tract, resulting in circulatory collapse with likely fatal consequences. As such, stent type valve prostheses are presently generally not suitable for use as a replacement mitral valve.
Mitral valve replacement techniques have also generally advocated removal of the native valve prior to location of the replacement mitral valve prosthesis. This is a technically extremely challenging task associated with the potentially fatal complication of profound mitral regurgitation that may not be adequately addressed by the subsequent valve replacement. The lack of an effective mitral valve may lead to overwhelming hemodynamic instability that may not be tolerated by the already compromised left ventricle and overwhelming pulmonary oedema may rapidly result.
Known stent based aortic valves are also not generally repositionable and therefore precise placement is difficult. This could result in important structures such as the coronary arteries being compromised as a result. Moreover, post-stenotic dilatation of the aorta may result in imprecise apposition of current stent based aortic valves, resulting in significant paravalvular leaks. For the same reason, current stent based aortic valves are typically not recommended for the treatment of pure aortic regurgitation. Current stent based aortic valves are also typically subject to fatigue and resultant fracture.
Further, various previously proposed replacement heart valve prostheses are relatively bulky and are thus not suited for percutaneous delivery using small diameter catheters, with more invasive larger catheters being required.